The treatment of paper-making fibers to render paper made therefrom stronger when wet has a long history of development. Many different polymeric materials have been utilized for this purpose. Among such polymers are the amino-aliphatic polymers set forth in U.S. Pat. No. 2,729,560 wherein there is disclosed the hypochlorite treatment of acrylamide polymers whereby 20-80% of the amide groups thereof are converted to amino groups. U.S. Pat. No. 2,890,978, on the other hand, discloses a similar hypochlorite process wherein less than 15% of the amide groups are degraded to amino groups and whereby dry strength is imparted to paper when the polymer produced is applied thereto. Both of these patents teach the addition to fibers or finished paper of polymers which are the final degradation products of the Hoffmann reaction of acrylamide polymers with hypochlorite or chlorine under alkaline conditions.
Additionally, U.S. Pat. No. 3,929,744 discloses the reaction of a sterically hindered amide polymer with a hypochlorite to produce an isocyanate - functional polymer useful as a coating composition after cross-linking with a polyol or polyamine, while U.S. Pat. No. 4,301,257 teaches a similar polymer produced by reacting a low molecular weight chlorinated acrylamide polymer with a tertiary amine having a pKa value of over 7 in an inert solvent. Twenty to one hundred percent of the amide groups of the charge polymer are said to be converted to isocyanate groups. The maximum molecular weight of the polymer is said to be about 10,000.
Processes of making polyfunctional N-chloroamide derivatives of acrylamide homo and copolymers are taught by U.S. Pat. Nos. 4,356,289 and 4,357,447 wherein chlorine in a dilute aqueous suspension of a mineral acid at 0.degree.-40.degree. C. is employed. German Pat. No. 2,931,572 also teaches such a chlorination process. Again, very low molecular weight polymers are produced.
Belgium Pat. No. 878,978 is similar to U.S. Pat. No. 4,301,257, discussed above, wherein amide-containing acrylic polymers are chlorinated and rearranged to isocyanate groups in the presence of tertiary amines. The polymers are of very low molecular weight.
Japanese Pat. No. 57,158,203 (82,158,203) discloses the reaction of polyacrylamide with a hypochlorite salt (or Cl.sub.2 and alkali) to provide isocyanate groups and then with a diamine to provide amino group-containing polymers useful for paper strengthening and is therefore also similar to U.S. Pat. No. 4,301,257.
Hahn et al. Angew, Makromol, Chem., 50, (1), 53-65, (1976) teaches the chlorination of polyacrylamides with HoCl, Me.sub.3 COCl and Cl.sub.2 O to provide chlorinated secondary amides wherein the recurring unit in the polymer is ##STR1## R.sup.5 being alkyl or aryl.
Water-in-oil polyacrylamide emulsions are chlorinated in U.S. Pat. No. 4,090,992 whereby said emulsions, containing less than 40% of water, are contacted with chlorine gas at -20.degree. C.
The present invention differs from the teachings of U.S. Pat. Nos. 2,729,560, and 2,890,978 disclosed above in that the compositions which are prepared by the present novel process and applied to paper are N-chloro substituted acrylamide polymers whereas the products of the '560 and '978 patents are amino-containing products. The prior art products contain amide, amino and carboxylate groups, but no N-chloro groups. The present invention further resides in the addition of the N-chloro group containing polymers to paper pulp or paper sheet followed by a reaction or series of reactions which cross-link the polymer and thus link the polymer to the paper fibers via the N-chloro functional groups. Furthermore, wet strength is developed in paper using products of the present invention containing as little as 1% N-chloroamide groups whereas '560 teaches that at least 20% of the amide groups must be converted to amino groups before wet strength is achieved in paper.
The teachings of U.S. Pat. No. 3,929,744 also are distinguishable from the instant invention in that the '744 patent focuses on the production of isocyanate group containing polymers. The polymers are not disclosed as paper additives, but coating compositions. Although isolatable, the chlorinated products are very unstable and water-insoluble.
U.S. Pat. No. 4,301,257, and, by similarity, Belgian Pat. No. 878,978 and Japanese Pat. No. 57,158,203, also teach the production of isocyanate group containing polymers as coatings. The products are void of any N-chloro groups and are not taught as functional for combining with cellulosic fibers for imparting wet strength to paper. The products produced by these prior art processes are water-insoluble and solvent soluble whereas the products of the present invention are water-soluble and solvent insoluble.
The Hahn, et al products are secondard amides and, as such, are not anticipatory of the instant products or disclosed as useful in paper-making.
The above-disclosed patents directed to the reaction of chlorine gas with acrylamide polymers using dilute aqueous suspensions in mineral acid, i.e., U.S. Pat. Nos. 4,356,289, 4,357,447 and German Pat. No. 2,931,572 are closely related to the invention disclosed herein but are distinguishable therefrom in many ways.
Primarily, the products of the prior art are almost insoluble in water, but have good solubility in organic solvents whereas, as mentioned above, our products are totally soluble in water and totally insoluble in almost all organic solvents. Secondarily, the products of the prior art are limited to a molecular weight of 10,000 whereas our products may have molecular weights as high as 1,000,000, or more.
U.S. Pat. No. 4,090,992 is the closest prior art known to the instant inventors which deals with the treatment of polymer emulsions with chlorine gas. This patent, however, labors under the misconception that the water content of the water-in-oil emulsion must be less than 40%, by weight, based on the amount of polymer and water in the emulsion. The patentee postulates that it is first necessary to remove any excess water from the emulsion before the contact with chlorine gas can be effective because if such is not accomplished, coagulation or agglomeration of the polymer results and the stability of the system fails. Furthermore, the prior art emulsion is dried by vacuum evaporation and requires a "breaker" surfactant added to the water used before a solution of the polymer will form. The patentee then adds chlorine to the dried emulsion, see Example 10, over 1 hour to give a total of 0.04 mole of chlorine which calculates to 9.5 moles of chlorine per 100 moles of polymer. At this stage, the procedure of the patent results in little, if any, reaction of the chlorine with the polymer. In reality, a solution of the chlorine in the oil of the emulsion results, which chlorine is released on subsequent warming. Following the chlorine addition, the patent discloses adding caustic soda. Sodium hypochlorite is thereby formed and reacts at high temperature according to the Hoffman reaction and the subsequent formation of an amino group containing polymer. The crux of the patent is that if the emulsion is not dehydrated, the chlorine gas, the sodium hydroxide emulsion and the breaker surfactant cannot be added. Not once in his disclosure does the patentee indicate that the charge polymer has been chlorinated.
It has now been found that chlorination of the acrylamide polymer can be accomplished without prior dehydration and is, in fact, more rapid than under dry conditions. The presence of water may be a necessary adjunct to the chlorination reaction.